Enable parallel locator queries

[hannahhoward]

What

For store protocol, we end up going to the blob index table on every locate call. It would be great if these calls could be parallelized. Fortunately, they can be with the indexing service.

In fact, there's a WIP PR to do so: #36

The work required here is just to make sure tracing is done properly (since the calls are now async) and probably to add a couple tests for the extracted batcher